1. Field of the Invention
The present invention relates to a radio communication apparatus and a transmission method, and more particularly to a radio communication apparatus and transmission method that are suitable for transmitting/receiving variable length RLC sub-PDU data in an RLC layer which belongs to a layer 2 constituting a protocol layer of radio communication, and are suitable for assembling RLC PDU correctly from the retransmitted RLC Sub-PDU data.
2. Description of the Related Art
A W-CDMA system is now widely used as a third generation (3G) radio communication system, and the standard called HSDPA (High-Speed Downlink Packet Access), where the speed of W-CDMA data communication is further increased (maximum 14 Mbps), is now being commercialized. HSDPA, which is an improved version of the 3G system, is also called 3.5G. HSDPA is being standardized by 3GPP (Third Generation Partnership Project), which is a 3G standardization group (see “3GPP TS 25.322”).
The features of HSDPA are (1) sharing one physical channel by a plurality of mobile terminals (UE) based on time division, (2) automatically selecting a faster modulation method and encoding method depending on the radio wave status, and (3) using a hybrid ARQ which is a combination of retransmission control (ARQ) and correction encoding processing.
FIG. 1 is a diagram depicting the layer 2 (Layer 2) of a protocol architecture corresponding to HSDPA. The layer 2 is divided into sub-layers, that is MAC (Medium Access Control)-hs, Mac-d and RLC (Radio Link Control).
FIG. 2 is a diagram depicting a format of a RLC PDU (Protocol Data Unit). The RLC PDU in FIG. 2 is an RLC PDU (this may also be called RLC AMD PDU in some cases, and the RLC PDU used herein below is RLC AMD PDU, AMD: Acknowledge Mode Data) that can control data transmission confirmation and data retransmission, and is comprised of a D/C bit for distinguishing user data and control data, a sequence number (SN) for indicating a sequence of RLC PDUs, a polling bit P for indicating the presence of a transmission confirmation request, an HE (Header Extension) type area for indicating the extended information of the user data, a Length Indicator LI, an E bit, a data storage area Data, and a padding bit PAD or a piggybacked STATUS PDU.
The data size of RLC PDU is fixed to 42 oct, 82 oct or 122 oct (1 oct (octet) is 8 bits), for example, and is not changed during communication. RLC PDU is identified by the sequence number SN, and a number in the 0 to 4095 range, for example, is assigned to the sequence number SN.
In the RLC in FIG. 1, the transmission data RLC SDU (Service Data Unit) from the higher layer at the RLC transmission side is divided into a plurality of RLC PDUs, to which a sequence number SN, to identify each PLC PDU, is assigned, and each PLC PDU is transmitted to the MAC-d layer at a lower level.
The RLC reception side, which received the RLC PDUs from the MAC-d layer at a lower level, rearranges the RLC PDUs in the sequence of the sequence numbers SN, connects them to assemble an RLC SDU, and transmits the RLC SDU to the higher layer. If a sequence number SN is missing, a retransmission of the RLC PDU corresponding to this missing SN is requested.
In preparation for the case when a retransmission is requested, the RLC transmission side must keep the transmitted RLC PDU in a buffer (memory) until a transmission confirmation notice is received from the RLC reception side. The RLC reception side has a buffer (memory) for storing a number of RLC PDUs required for assembling an RLC SDU, and assembles the RLC SDU when all the RLC PDUs required for assembling a RLC SDU are stored.
The transmission confirmation control is performed by the RLC transmission side setting the polling bit included in the RLC PDU P to “1”. If the reception side receives an RLC PDU of which polling bit P is set to “1”, the reception side confirms no sequence number SN of the RLC PDUs received thus far is missing, and transmits STATUS PDU (ACK) if no sequence number is missing. If missing, the reception side responds with STATUS PDU (NACK). The reception side has a timer to prevent the frequent generation of STATUS PDU responses after one STATUS PDU response, and if NACK is used to advise that information is missing, the timer is started at that point, and the next STATUS PDU response is not performed until the timer is up.
The transmission side starts the timer after a transmission confirmation request is transmitted, and if a STATUS PDU response is not received from the reception side before the timer is up, it is judged that a data loss occurred in a radio band, and transmits the RLC PDU in which the polling bit P is set to “1” again. If a STATUS PDU (ACK) is received, it is judged that RLC PDUs have been transmitted/received normally thus far, and if a retransmission request by STATUS PDU (NACK) is received, then the RLC PDU of which retransmission request is requested is transmitted again.
Japanese Patent Application Laid-Open No. 2006-20044 discloses a memory management method that divides a MAC-hs PDU, which is a variable length data, into RLC PDC units in an MAC-hs sub-layer, and stores each RLC PDU along with a sequence number in a shared memory (buffer) so that a complicated memory control need not be used, and an increase in memory volume can be suppressed.